1. Field of the Invention
The invention relates to a multi-chip module, more particularly to a miniaturized multi-chip module suitable for application to wireless transmission devices and to a method for manufacturing the same.
2. Description of the Related Art
FIG. 1 illustrates a conventional multi-chip module 20 for application to a wireless transmission device (not shown). The multi-chip module 20 includes a plurality of integrated circuit chips 21, a module substrate 22, and a plurality of solder balls 23.
One of the integrated circuit chips 21 has a wireless transmission capability.
The module substrate 22 has a first surface 221, a second surface 222 opposite to the first surface 221, a plurality of first solder pads 223 formed on the first and second surfaces 221, 222, and a plurality of second solder pads 224 formed on the first surface 221 and connected electrically to the first solder pads 223.
In general, the solder balls 23 are planted onto the second solder pads 224 using a ball-planting machine (not shown), and configure the multi-chip module 20 into a ball grid array (BGA) package. The solder balls 23 serve as external electrical connections for the integrated circuit chips 21 when the multi-chip module 20 is mounted on a circuit board (not shown) of a target device (not shown) so as to provide the latter with a wireless transmission capability.
FIGS. 2A to 2E illustrate consecutives steps of a method for manufacturing the conventional multi-chip module 20 of FIG. 1. The manufacturing method includes the following steps:
a) Referring to FIG. 2A, the module substrate 22 is formed. The module substrate 22 has first and second surfaces 221, 222, the first solder pads 223 formed on the first and second surfaces 221, 222, and the second solder pads 224 formed on the first surface 221. The electrical connections between the first and second solder pads 223, 224 are not illustrated for the sake of brevity.
b) Referring to FIG. 2B, a first one of the integrated circuit chips 21 is mounted fixedly on the second surface 222 of the module substrate 22 at corresponding ones of the first solder pads 223 through conventional solder techniques.
c) Referring to FIG. 2C, the module substrate 22 is turned such that the first surface 221 thereof faces upwardly and, referring to FIG. 2D, second and third ones of the integrated circuit chips 21 are mounted fixedly on the first surface 221 of the module substrate 22 at corresponding ones of the first solder pads 223 through conventional solder techniques.
d) Finally, referring to FIG. 2E, the solder balls 23 are planted onto the second solder pads 224, respectively, to complete the multi-chip module 20.
As shown in FIG. 2F, the solder balls 23 are to be registered with and to be connected to corresponding solder pads 11 on a circuit board 10 of a target device (not shown). When mounted, the multi-chip module 20 provides the target device with a wireless transmission capability.
The following are some of the drawbacks attributed to the use of the solder balls 23 in the conventional multi-chip module 20:
1. Referring again to FIG. 1, it is assumed that the integrated circuit chips 21 on the first surface 221 of the module substrate 22 have a maximum height of 0.4 mm. Therefore, the diameter of each of the solder balls 23 should be larger than 0.4 mm. In practice, the diameter of the solder balls 23 is chosen to be at least 0.5 mm. In addition, there should be a gap of at least 0.4 mm between adjacent ones of the solder balls 23 to avoid occurrence of short circuiting between the adjacent solder balls 23. Under these conditions, if the size of the module substrate 22 is 10×10 mm, and if solder balls 23 are not to be provided at four corners of the first surface 221 of the module substrate 22, each side of the first surface 221 of the module substrate 22 can accommodate a maximum of nine solder balls 23. In other words, since the module substrate 22 can only accommodate a total of thirty-six solder balls 23, the number of available external electrical connections for the integrated circuit chips 21 cannot exceed thirty-six. In view of the ever-increasing complexity and growing functionality of the integrated circuit chips 21, the required number of external electrical connections for the chips 21 could eventually exceed thirty-six, which necessitates the use of a larger module substrate 22. However, a larger module substrate 22 goes against the trend toward miniaturization of electronic devices.
2. As shown in FIG. 2F, when the conventional multi-chip module 20 is mounted on the circuit board 10 of the target device, the solder balls 23 are subjected to thermal and compression stresses that can result in non-uniform deformation of the solder balls 23 and in the possibility of short-circuiting between adjacent ones of the solder balls 23.
3. The ball-planting machine for planting the solder balls 23 is an expensive piece of equipment. Purchasing the ball-planting machine results in higher capital costs for manufacturers of the conventional multi-chip module 20. While the ball-planting operation could be contracted to other manufacturers, this would entail a longer production cycle.